Remove outdated references to other tutorial

tutorials/shaders/advanced_postprocessing.rst

@@ -11,18 +11,11 @@ In particular, it will explain how to write a post-processing shader that
 uses the depth buffer. You should already be familiar with post-processing
 generally and, in particular, with the methods outlined in the :ref:`custom post-processing tutorial <doc_custom_postprocessing>`.
 
-In the previous post-processing tutorial, we rendered the scene to a :ref:`Viewport <class_Viewport>`
-and then rendered the Viewport in a :ref:`SubViewportContainer <class_SubViewportContainer>`
-to the main scene. One limitation of this method is that we could not access the
-depth buffer because the depth buffer is only available in shaders and
-Viewports do not maintain depth information.
-
 Full screen quad
 ----------------
 
-In the :ref:`custom post-processing tutorial <doc_custom_postprocessing>`, we
-covered how to use a Viewport to make custom post-processing effects. There are
-two main drawbacks of using a Viewport:
+One way to make custom post-processing effects is by using a viewport. However,
+there are two main drawbacks of using a Viewport:
 
 1. The depth buffer cannot be accessed
 2. The effect of the post-processing shader is not visible in the editor